Many of the existing volumetric pumps known in the art, such as the ones described in GB860616, U.S. Pat. No. 5,312,233 and EP1817499, comprise a single piston in a chamber. The piston instroke fills the piston chamber with a specific amount of a fluid (filling phase) while the piston outstroke releases said amount of fluid out of the chamber (releasing phase). Unlike other pumps where the piston and the valve system are driven independently from each other, these pumps are driven by a mechanism which couples the piston strokes with the movement of the valve system. This guarantees that the valve commutations always occur at the end of a stroke of the volumetric pump avoiding possible back flow. A major drawback of these pumps is that the flow rate of the released fluid is intermittent as no fluid is expelled during the piston instroke.
International application No. WO2006056828, which is incorporated hereing by reference, describes a volumetric pump comprising first and second pistons whose movements inside their respective chambers is synchronized such that a specific amount of fluid is sucked in during the instroke of one piston while the same amount of fluid is expelled during the outstroke of the other piston. The first and second pistons are arranged along a longitudinal axis inside first and second hollow cylindrical parts (chambers) which are assembled end-to-end facing each other to form a housing. A valve disc (valve system), which comprises an inlet and outlet port connected respectively to an inlet and outlet T-shaped channel, is mounted between the first and second piston inside the housing and is arranged to be animated by a combined bidirectional linear and angular movement which couples the piston strokes with the movement of the valve system. More precisely, the linear movement of the disc produces a to-and-fro sliding of the cylindrical housing along the axis of the pistons causing an alternate instroke of the first and second pistons followed by an alternate outstroke of the first and second pistons inside their respective chambers while its angular movement synchronizes the first piston chamber filling phase with the second piston releasing phase. This synchronization is achieved by the inlet and outlet T-shaped channel located inside the valve disc which connects alternately the inlet port to the first and second chamber, and the first and second chamber to the outlet port when said channels overlap alternately an inlet aperture and an outlet aperture located across the diameter of both cylindrical parts adjacent to the lateral sides of said disc. The flow of the fluid released by this pump is quasi-continuous.
However, the flow rate of the fluid delivered by this pump is irregular given that it is directly dependent on the distance travelled by each piston inside its respective cylinder. In fact, the pressure produced when the first and second pistons are alternately in their releasing phase varies according to a sinusoidal curve. As a result, the flow rate of the liquid released by the pump progressively increases as one of the two pistons begins its outstroke until said piston reaches the middle of its stroke. Subsequently, the flow rate progressively decreases as the piston reaches the end of its stroke. At this specific time, both pistons are immobilized for a short time to ensure no pumping movement when the valves are commuting (idle time) before beginning another cycle. Thus, no liquid is released during the idle time.
A major drawback of this volumetric pump is that the inlet and outlet aperture, arranged to be aligned alternately with the inlet and outlet T-shaped channel, are located across the diameter of both cylindrical parts adjacent to the lateral sides of the valves disc. As a result, the volume reduction of the first and second chamber is limited to the size of the apertures below which it would be insufficient to guarantee a normal flow delivery.
In addition, the inner construction of this volumetric pump make it difficult to integrate further chambers in parallel which could provide a solution for obtaining a continuous and steadier flow rate when working at a certain pressure.